Hydrocarbons classified as aromatics have enjoyed increasing demand in the marketplace due principally to their versatility as gasoline blending components. In addition, they can also be used as components in the production of various petrochemical compounds. This is particularly true in the case of benzene. Benzene represents the building block for the direct or indirect manufacture of well over 250 separate products or product classifications. Over the past few years, the annual benzene demand in the United States alone has ranged from 1.5 to 1.9 billion gallons. Worldwide, the annual consumption of benzene has ranged from 3.5 to 4.2 billion gallons. Historically, the major consumption of benzene has been in the production of ethylbenzene, cumene and cyclohexane. The principal use of ethylbenzene is to produce styrene by, for example, steam hydrogenation. Significant amounts of benzene are also consumed in the manufacture of aniline, detergent alkylate, and maleic anhydride.
At the present time, most of the total aromatics produced in the U.S. come from catalytic reforming of hydrocarbons. Typical reforming reactions include the dehydrogenation of naphthenes to produce aromatics, dehydrocyclization of paraffins directly to aromatics, the hydrocracking of long-chained paraffins into lower boiling, normally liquid material, and the isomerization of paraffins. In catalytic reforming of hydrocarbons, fresh liquid hydrocarbons boiling within the gasoline or naphtha boiling range are reacted with hydrogen in the presence of a catalyst comprising a Group VIII noble metal on a porous carrier at conditions which promote the conversion of naphthenes and paraffins to aromatic hydrocarbons.
Catalytic reforming is primarily an endothermic process effected in a plurality of reaction zones having interstage heating therebetween. The operation is effected primarily in vapor phase at temperatures of up to 1200.degree. F. Other operating conditions include a pressure of about 20 to 1000 psig, a liquid hourly space velocity of about 0.2 to 10, and a hydrogen to hydrocarbon mole ratio of about 0.5:1 to 20:1.
The prior art is replete with catalytic reforming processes using a variety of schemes. For example, U.S. Pat. No. 3,664,949 (issued to Petersen et al.) discloses a process for reforming a petroleum hydrocarbon feedstock that boils within the range of about 120.degree. to 500.degree. F. and is selected from a group consisting of virgin naphthas, cracked naphthas, catalytic gasolines, coker naphthas, and mixtures thereof. In this process, the above-described feedstock is contacted in a reactor system consisting of two reactors in the presence of hydrogen and under reforming conditions with a catalyst in each reactor comprising a Group VIII noble metal and a co-catalytic solid support comprising mordenite. Another example of a catalytic reforming process can be found in U.S. Pat. No. 3,864,241 (issued to Rausch). The Rausch patent discloses a process for catalytically reforming a gasoline fraction comprising contacting the fraction with a catalytic composite comprising a combination of a platinum group component, a tin component, and a halogen component.
Processes that seek to maximize the production of benzene take at least a portion of the catalytic reformate containing alkylaromatics and react it in a dealkylation zone in the presence of hydrogen at conditions selected to dealkylate alkyl-substituted aromatic hydrocarbons. Thus, toluene and mixed xylenes are dealkylated for maximum benzene production, or toluene is transalkylated to maximize production of both benzene and mixed xylenes.
U.S. Pat. No. 3,197,523 is illustrative of a hydrodealkylation process. In this process, a feedstock comprising toluene, mixed xylenes, ethylbenzene, mixed diethylbenzenes, and various alkyl-substituted naphthalenes is reacted in the presence of a catalyst containing at least one oxide of tin, titanium, and zirconium combined with at least one oxide in chromium, molybdenum and tungsten at conditions including temperatures of about 1000.degree. to 1500.degree. F. and pressures of about 300 to 1000 psig.
U.S. Pat. No. 4,157,355 (issued to Addison) discloses an integrated catalytic reforming and hydrodealkylation process wherein a liquid phase of a catalytic reforming effluent is passed to a catalytic hydrodealkylation zone, the products of which are separated into a hydrogen-rich vapor phase and a liquid aromatic-containing phase. The hydrogen-rich vapor phase is then recycled to the catalytic reforming zone, and the aromatic-containing liquid phase is sent to a fractionator wherein a benzene-rich stream is recovered.
The prior art integrated processes have several disadvantages. First, in the prior art process, the hydrogen-rich gas contains some light hydrocarbons which are carried forward to the hydrodealkylation unit and which will crack to form methane in the hydrodealkylation unit, thereby increasing the overall hydrogen consumption. Second, in the prior art processes, a significant amount of benzene, hydrogen, and some methane can be lost through venting of purge gases in the hydrodealkylation unit. Third, the use of a catalytic hydrodealkylation unit has the disadvantage of process shutdowns required for catalyst replacement.
There is a need for an integrated catalytic reforming/hydrodealkylation process that maximizes the recovery of benzene and uses hydrogen more efficiently.